In an RKE cluster (without a cloud provider configured) is there a way to have Rancher automatically removed worker nodes from the cluster after it's been unavailable/unreachable for X amount of time?

nope. Without an external cloud controller to verify that the instance has actually been deleted, it is generally considered unsafe to assume that a node that is NotReady is gone for good.

Ok thanks. I'm using the cluster-autoscaler in Rancher to scale up/down worker nodes in an EC2 autoscale group. My problem is that when nodes are scaled down and killed in the ASG they're left hanging around in Rancher until I manually delete them. I know this is basically working as intended but I was hoping for a way around it without configuring a cloud provider, it'll be tricky in our environment (IAM permissions are locked down quite a bit).

yeah, I think you’re intended to use the AWS cloud controller to handle that

Awesome thank you, good find. If I were to try the cloud provider route again, is it possible to add/remove a cloud provider to an existing cluster without causing any problems?

you can generally add one after the fact, but not change it.